net, wireless, mwifiex: Fix mem leak in mwifiex_update_curr_bss_params()

If kmemdup() fails we leak the memory allocated to bss_desc.
This patch fixes the leak.
I also removed the pointless default assignment of 'NULL' to 'bss_desc'
while I was there anyway.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>