mwifiex: don't leak stashed beacon buffer on reset

When removing or resetting an mwifiex device, we don't remember to free
the saved beacon buffer. Use the (somewhat misleadingly-named)
mwifiex_free_priv() helper to handle this.

Noticed by kmemleak during tests:

echo 1 > /sys/bus/pci/devices/.../reset

unreferenced object 0xffffffc09d034a00 (size 256):
...
  backtrace:
    [<ffffffc0003cdce4>] create_object+0x228/0x3c4
    [<ffffffc000c0b9d8>] kmemleak_alloc+0x54/0x88
    [<ffffffc0003c0848>] __kmalloc+0x1cc/0x2dc
    [<ffffffbffc1500c4>] mwifiex_save_curr_bcn+0x80/0x308 [mwifiex]
    [<ffffffbffc1516b8>] mwifiex_ret_802_11_associate+0x4ec/0x5fc [mwifiex]
    [<ffffffbffc15da90>] mwifiex_process_sta_cmdresp+0xaf8/0x1fa4 [mwifiex]
    [<ffffffbffc1411e0>] mwifiex_process_cmdresp+0x40c/0x510 [mwifiex]
    [<ffffffbffc13b8f4>] mwifiex_main_process+0x4a4/0xb00 [mwifiex]
    [<ffffffbffc13bf84>] mwifiex_main_work_queue+0x34/0x40 [mwifiex]

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>