mfd: si476x: Fix i2c warning

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]

Acked-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>