mwifiex: Remove unused 'pm_flag' variable

mwifiex_sdio_resume() intializes pm_flag, just like
mwifiex_sdio_suspend(), but does not use it. Compiling with W=1 gives
the following warning, fix it.
mwifiex/sdio.c: In function ‘mwifiex_sdio_resume’:
mwifiex/sdio.c:234:16: warning: variable ‘pm_flag’ set but not used [-Wunused-but-set-variable]

sdio_get_host_pm_caps() is just an acessor, so the call to it is safe
to remove. The unused variable seems to be present since
5e6e3a92 which introduced mwifiex_sdio_resume().

Fixes: 5e6e3a92 ("wireless: mwifiex: initial commit for Marvell mwifiex driver")
Cc: Bing Zhao <bzhao@marvell.com>
Cc: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>