[media] si476x: Fix some config dependencies and a compile warnings

radio-si476x depends on SND and SND_SOC, the mfd driver should select
REGMAP_I2C.
Also fix a small compile warning in a debug message:
drivers/mfd/si476x-i2c.c: In function ‘si476x_core_drain_rds_fifo’:
drivers/mfd/si476x-i2c.c:391:4: warning: field width specifier ‘*’ expects argument of type ‘int’, but argument 4 has type ‘long unsigned int’ [-Wformat]

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>