mwifiex: fix NULL pointer dereference during suspend

This patch fixes below NULL pointer dereference observed in suspend
stress test. When scan is cancelled during system suspend, we may end
up aceesing "priv->scan_request" in corner case.

[ 3035.304682] BUG: KASAN: null-ptr-deref on address 0000000000000008
[ 3035.304704] Read of size 4 by task ksdioirqd/mmc2/1183
[ 3035.304744] CPU: 0 PID: 1183 Comm: ksdioirqd/mmc2 Tainted: G        W      3.18.0 #1169
[ 3035.304772] Call trace:
[ 3035.304825] [<ffffffc00020a520>] dump_backtrace+0x0/0x190
[ 3035.304864] [<ffffffc00020a6cc>] show_stack+0x1c/0x28
[ 3035.304901] [<ffffffc000b36db8>] dump_stack+0xa0/0xf8
[ 3035.304940] [<ffffffc00039c494>] kasan_report+0x120/0x4fc
[ 3035.304975] [<ffffffc00039b6b4>] __asan_load4+0x20/0x80
[ 3035.305546] [<ffffffbffc1f5aec>] mwifiex_check_next_scan_command+0x1a4/0x588 [mwifiex]
[ 3035.306091] [<ffffffbffc1f7aec>] mwifiex_handle_event_ext_scan_report+0x304/0x370 [mwifiex]
[ 3035.306735] [<ffffffbffc206bb8>] mwifiex_process_sta_event+0x6c0/0xf10 [mwifiex]
[ 3035.307200] [<ffffffbffc1e609c>] mwifiex_process_event+0x2f4/0x358 [mwifiex]
[ 3035.307612] [<ffffffbffc1e25c8>] mwifiex_main_process+0x3cc/0x80c [mwifiex]
[ 3035.307737] [<ffffffbffc2523a0>] mwifiex_sdio_interrupt+0x198/0x1c0 [mwifiex_sdio]
[ 3035.307785] [<ffffffc0008d9250>] process_sdio_pending_irqs+0x15c/0x1d4
[ 3035.307826] [<ffffffc0008d93f0>] sdio_irq_thread+0xd8/0x288

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