Staging: rtlwifi: Remove Unneeded funition halbtc_get_wifi_rssi()

In the function halbtc_get_wifi_rssi:

1.if and else are functionally identical as they assign same value to
  variable 'undec_sm_pwdb'.
2.Variable 'undec_sm_pwdb' not required to just return a single value.
3.Function not needed to just return a value and so removed the function
  and assigned value rtlpriv->dm.undec_sm_pwdb where the function
  halbtc_get_wifi_rssi() is called.

Signed-off-by: Bhanusree Pola <bhanusreemahesh@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>