mwifiex: remove redundant 'fw_load' completion structure

'add_remove_card_sem' semaphore already takes care of
synchronization for driver load and unload threads.
Hence there won't be a case when unload thread is waiting on
'wait_for_completion(fw_load)'.

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>