mwifiex: fix smatch errors

drivers/net/wireless/mwifiex/main.c +828 mwifiex_remove_card(52)
  error: potential null derefence 'priv'.
drivers/net/wireless/mwifiex/main.c +828 mwifiex_remove_card(52)
  error: we previously assumed 'priv' could be null (see line 820)
drivers/net/wireless/mwifiex/txrx.c +90 mwifiex_process_tx(24)
  error: potential null derefence 'local_tx_pd'.
drivers/net/wireless/mwifiex/sta_ioctl.c +766
mwifiex_rate_ioctl_set_rate_value(30)
  error: buffer overflow 'rate' 14 <= 14

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>