wil6210: Fix potential memory leaks on error paths

Fix missing memory deallocation on error paths in wil_write_file_wmi()
and wil_write_file_txmgmt().

Reported-by: Ahmed Tamrawi <ahmedtamrawi@gmail.com>
Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>