mwifiex: remove redundant signal handling code

1) The wrapper function mwifiex_get_signal_info() is unnecessary.
2) As noise and signal vaules in private structure already get
modified, we don't need to explicitly pass
"struct mwifiex_ds_get_signal" to get it filled.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>