si476x_core_send_command()'s 5th argument has never been called response.

This change must have occurred prior to the driver being Mainlined.

We're also taking the opportunity to bring the first description back
into line, making my OCD happy!

This fixes the following W=1 warning(s):

 drivers/mfd/si476x-cmd.c:264: warning: Function parameter or member 'resp' not described in 'si476x_core_send_command
 drivers/mfd/si476x-cmd.c:264: warning: Excess function parameter 'response' description in 'si476x_core_send_command'

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