mfd: si476x-i2c: Fix incorrectly documented function names

Fixes the following W=1 kernel build warning(s):

 drivers/mfd/si476x-i2c.c:360: warning: expecting prototype for si476x_drain_rds_fifo(). Prototype was for si476x_core_drain_rds_fifo() instead
 drivers/mfd/si476x-i2c.c:467: warning: expecting prototype for si476x_get_status(). Prototype was for si476x_core_get_status() instead
 drivers/mfd/si476x-i2c.c:484: warning: expecting prototype for si476x_get_and_signal_status(). Prototype was for si476x_core_get_and_signal_status() instead
 drivers/mfd/si476x-i2c.c:555: warning: expecting prototype for si476x_firmware_version_to_revision(). Prototype was for si476x_core_fwver_to_revision() instead
 drivers/mfd/si476x-i2c.c:619: warning: expecting prototype for si476x_get_revision_info(). Prototype was for si476x_core_get_revision_info() instead

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>