media: si470x: fix __be16 annotations

The annotations there are wrong as warned:
   drivers/media/radio/si470x/radio-si470x-i2c.c:107:35: warning: cast to restricted __be16
   drivers/media/radio/si470x/radio-si470x-i2c.c:107:35: warning: cast to restricted __be16
   drivers/media/radio/si470x/radio-si470x-i2c.c:107:35: warning: cast to restricted __be16
   drivers/media/radio/si470x/radio-si470x-i2c.c:107:35: warning: cast to restricted __be16
   drivers/media/radio/si470x/radio-si470x-i2c.c:129:24: warning: incorrect type in assignment (different base types)
   drivers/media/radio/si470x/radio-si470x-i2c.c:129:24:    expected unsigned short [unsigned] [short] <noident>
   drivers/media/radio/si470x/radio-si470x-i2c.c:129:24:    got restricted __be16 [usertype] <noident>
   drivers/media/radio/si470x/radio-si470x-i2c.c:163:39: warning: cast to restricted __be16
   drivers/media/radio/si470x/radio-si470x-i2c.c:163:39: warning: cast to restricted __be16
   drivers/media/radio/si470x/radio-si470x-i2c.c:163:39: warning: cast to restricted __be16
   drivers/media/radio/si470x/radio-si470x-i2c.c:163:39: warning: cast to restricted __be16

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