[media] m920x: let GCC see 'ret' is used initialized

[media] m920x: let GCC see 'ret' is used initialized

Since commit 7543f344e9b06afe86b55a2620f5c11b38bd5642 ("[media] m920x:
factor out a m920x_write_seq() function") building m920x.o triggers this
GCC warning:
    drivers/media/usb/dvb-usb/m920x.c: In function ‘m920x_probe’:
    drivers/media/usb/dvb-usb/m920x.c:91:6: warning: ‘ret’ may be used uninitialized in this function [-Wuninitialized]
This warning is caused by m920x_write_seq(), which is apparently inlined
into m920x_probe(). It is clear why GCC thinks 'ret' may be used
uninitialized. But in practice the first seq->address will always be
non-zero when this function is called. That means we can change the
while()-do{} loop into a do{}-while() loop. And that suffices to make
GCC see that 'ret' will not be used uninitialized.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>