staging: rtl8712: fix potential leaks in r8712_set_key()

Fix potential leaks in the error paths of r8712_set_key(). In case the
algorithm specific checks fail, 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 144370, 144371.

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