mfd: si476x-i2c: Add description for si476x_core_fwver_to_revision()'s arg 'func'

Kerneldoc syntax is used, but not complete.

Descriptions are required for all arguments.

Fixes the following W=1 build warning:

 drivers/mfd/si476x-i2c.c:550: warning: Function parameter or member 'func' not described in 'si476x_core_fwver_to_revision'

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