staging: ks7010: refactor ks_wlan_set_rx_gain function

This commit refactors ks_wlan_set_rx_gain function to
improve readability:
    - error condition is handling the error to avoid an 'else'
    - ternary operator is used to clean if-else block assignment.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>