[media] radio-miropcm20: fix a compilation warning

drivers/media/radio/radio-miropcm20.c: In function 'sanitize':
drivers/media/radio/radio-miropcm20.c:216:3: warning: comparison is always false due to limited range of data type [-Wtype-limits]
   if (p[i] < 32 || p[i] >= 128) {
   ^

As p is declared as a char array, it is signed. So, it can never
be bigger than 127.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>