wifi: mwifiex: remove unused evt_buf variable

clang with W=1 reports
drivers/net/wireless/marvell/mwifiex/11h.c:198:6: error: variable
  'evt_buf' set but not used [-Werror,-Wunused-but-set-variable]
        u8 *evt_buf;
            ^
This variable is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230329131444.1809018-1-trix@redhat.com