cx18: use macros instead of static const vars

cx18: use macros instead of static const vars

Gcc 6.1 now complains about unused vars:

drivers/media/pci/cx18/cx18-driver.h:497:18: warning: 'vbi_hblank_samples_50Hz' defined but not used [-Wunused-const-variable=]
 static const u32 vbi_hblank_samples_50Hz = 284; /* 4 byte EAV + 280 anc/fill */
                  ^~~~~~~~~~~~~~~~~~~~~~~
drivers/media/pci/cx18/cx18-driver.h:496:18: warning: 'vbi_hblank_samples_60Hz' defined but not used [-Wunused-const-variable=]
 static const u32 vbi_hblank_samples_60Hz = 272; /* 4 byte EAV + 268 anc/fill */
                  ^~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/media/pci/cx18/cx18-cards.c:25:0:
drivers/media/pci/cx18/cx18-driver.h:497:18: warning: 'vbi_hblank_samples_50Hz' defined but not used [-Wunused-const-variable=]
 static const u32 vbi_hblank_samples_50Hz = 284; /* 4 byte EAV + 280 anc/fill */
                  ^~~~~~~~~~~~~~~~~~~~~~~
drivers/media/pci/cx18/cx18-driver.h:496:18: warning: 'vbi_hblank_samples_60Hz' defined but not used [-Wunused-const-variable=]
 static const u32 vbi_hblank_samples_60Hz = 272; /* 4 byte EAV + 268 anc/fill */
                  ^~~~~~~~~~~~~~~~~~~~~~~
drivers/media/pci/cx18/cx18-driver.h:495:18: warning: 'vbi_active_samples' defined but not used [-Wunused-const-variable=]
 static const u32 vbi_active_samples = 1444; /* 4 byte SAV + 720 Y + 720 U/V */
                  ^~~~~~~~~~~~~~~~~~

In this specific case, this is somewhat intentional, as those
values are actually used in parts of the driver. The code assumes
that gcc optimizer it and not actually create any var, but convert
it to immediate access at the routines.

Yet, as we want to shut up gcc warnings, let's use #define, with
is the standard way to store values that will use assembler's
immediate access code.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>