mwifiex: Removed unused mwifiex_private* 'priv' variable

Commit bec568ff removed the last remaining usage of struct
mwifiex_private* priv in mwifiex_fw_dpc(), by removing the call to
mwifiex_del_virtual_intf().
Compiling mwifiex/ with W=1 gives the following warning, fix it.
mwifiex/main.c: In function ‘mwifiex_fw_dpc’:
mwifiex/main.c:520:26: warning: variable ‘priv’ set but not used [-Wunused-but-set-variable]

Fixes: bec568ff ("mwifiex: failure path handling in mwifiex_add_virtual_intf()")
Cc: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>