mwifiex: fix bug in mwifiex_save_curr_bcn()

Since timestamp in beacon buffer keeps changing all the time,
the memcmp check in mwifiex_save_curr_bcn() is redundant.
Remove that memcmp check and also avoid freeing and allocation
of buffer if required beacon buffer size is same as previous one.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>