staging: rtl8188eu: fix potential leak in rtw_set_key()

Fix a potential leak in the error path of rtw_set_key(). In case the requested
algorithm is not supported by the driver, the function returns without
enqueuing or freeing the already allocated command and parameter structs. Use
a centralized exit path and make sure that all memory is freed correctly.
Detected by Coverity - CID 1077716, 1077717.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>