mwifiex: Remove unused 'sta_ptr' variable

Commit 429d90d2 introduced mwifiex_cmd_tdls_oper() which initializes
struct mwifiex_sta_node* sta_ptr, but does not use it. Compiling with W=1 gives
the following warning, fix it.
mwifiex/sta_cmd.c: In function ‘mwifiex_cmd_tdls_oper’:
mwifiex/sta_cmd.c:1732:27: warning: variable ‘sta_ptr’ set but not used [-Wunused-but-set-variable]

Fixes: 429d90d2 ("mwifiex: add cfg80211 tdls_oper handler support")
Cc: Avinash Patil <patila@marvell.com>
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>