mwifiex: Remove unused 'adapter'variable

Commit 3935ccc1 introduced mwifiex_tm_cmd() which initializes
struct mwifiex_adapter* adapter, but doesn't use it.
Compiling with W=1 gives the following warning, fix it.
mwifiex/cfg80211.c: In function ‘mwifiex_tm_cmd’:
mwifiex/cfg80211.c:3973:26: warning: variable ‘adapter’ set but not used [-Wunused-but-set-variable]

Fixes: 3935ccc1 ("mwifiex: add cfg80211 testmode support")
Cc: Xinming Hu <huxm@marvell.com>
Cc: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>