mwifiex: don't call del_timer_sync() on uninitialized timer

syzbot is reporting that del_timer_sync() is called from
mwifiex_usb_cleanup_tx_aggr() from mwifiex_unregister_dev() without
checking timer_setup() from mwifiex_usb_tx_init() was called [1].

Ganapathi Bhat proposed a possibly cleaner fix, but it seems that
that fix was forgotten [2].

"grep -FrB1 'del_timer' drivers/ | grep -FA1 '.function)'" says that
currently there are 28 locations which call del_timer[_sync]() only if
that timer's function field was initialized (because timer_setup() sets
that timer's function field). Therefore, let's use same approach here.

[1] https://syzkaller.appspot.com/bug?id=26525f643f454dd7be0078423e3cdb0d57744959
[2] https://lkml.kernel.org/r/CA+ASDXMHt2gq9Hy+iP_BYkWXsSreWdp3_bAfMkNcuqJ3K+-jbQ@mail.gmail.com

Reported-by: syzbot <syzbot+dc4127f950da51639216@syzkaller.appspotmail.com>
Cc: Ganapathi Bhat <ganapathi.bhat@nxp.com>
Cc: Brian Norris <briannorris@chromium.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Acked-by: Ganapathi Bhat <ganapathi.bhat@nxp.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200821082720.7716-1-penguin-kernel@I-love.SAKURA.ne.jp

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