mfd: si476x-cmd: Add missing documentation for si476x_cmd_fm_rds_status()'s arg 'report'

Kerneldoc syntax is used, but not complete.

Descriptions are required for all arguments.

Fixes the following W=1 build warning:

 drivers/mfd/si476x-cmd.c:907: warning: Function parameter or member 'report' not described in 'si476x_core_cmd_fm_rds_status'

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