mwifiex: Fix request_irq() failure handling

It's been observed that request_irq() failure leads to a system crash
due to a bug in mwifiex driver.
When this failure happens, mwifiex_add_card() already takes care of
clearing and freeing adapter->card pointer. This patch removes the
redundant cleanup code causing crash.

Signed-off-by: Karthik D A <karthida@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>