[media] radio-bcm2048: fix signal of value

[media] radio-bcm2048: fix signal of value

As value can be initialized with a value lower than zero, change it
to int, to avoid those warnings:

drivers/staging/media/bcm2048/radio-bcm2048.c: In function 'bcm2048_rds_pi_read':
drivers/staging/media/bcm2048/radio-bcm2048.c:1989:9: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
  struct bcm2048_device *bdev = dev_get_drvdata(dev);  \
         ^
drivers/staging/media/bcm2048/radio-bcm2048.c:2070:1: note: in expansion of macro 'property_read'
 property_read(rds_pi, unsigned int, "%x")
 ^
drivers/staging/media/bcm2048/radio-bcm2048.c: In function 'bcm2048_fm_rds_flags_read':
drivers/staging/media/bcm2048/radio-bcm2048.c:1989:9: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
  struct bcm2048_device *bdev = dev_get_drvdata(dev);  \
         ^
drivers/staging/media/bcm2048/radio-bcm2048.c:2074:1: note: in expansion of macro 'property_read'
 property_read(fm_rds_flags, unsigned int, "%u")
 ^
drivers/staging/media/bcm2048/radio-bcm2048.c: In function 'bcm2048_region_bottom_frequency_read':
drivers/staging/media/bcm2048/radio-bcm2048.c:1989:9: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
  struct bcm2048_device *bdev = dev_get_drvdata(dev);  \
         ^
drivers/staging/media/bcm2048/radio-bcm2048.c:2077:1: note: in expansion of macro 'property_read'
 property_read(region_bottom_frequency, unsigned int, "%u")
 ^
drivers/staging/media/bcm2048/radio-bcm2048.c: In function 'bcm2048_region_top_frequency_read':
drivers/staging/media/bcm2048/radio-bcm2048.c:1989:9: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
  struct bcm2048_device *bdev = dev_get_drvdata(dev);  \
         ^
drivers/staging/media/bcm2048/radio-bcm2048.c:2078:1: note: in expansion of macro 'property_read'
 property_read(region_top_frequency, unsigned int, "%u")

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>