staging: rtl8188eu: check for kzalloc failure

Check for kzalloc failure and directly return from the error patch thus
simplifying the success path.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>