mwifiex: Remove unused 'chan_num' variable

Commit b5413e6b removed all uses of chan_num in mwifiex_config_scan().
Compiling mwifiex with W=1 gives the following warning, fix it.

mwifiex/scan.c: In function ‘mwifiex_config_scan’:
mwifiex/scan.c:830:6: warning: variable ‘chan_num’ set but not used [-Wunused-but-set-variable]

Fixes: b5413e6b ("mwifiex: increase the number of nodes in command pool")
Cc: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>