staging: wilc1000: return -ENOMEM when kmalloc failed

The driver is using -1 instead of the -ENOMEM defined macro to specify that
a buffer allocation failed.

Fixes smatch warning and similars:
drivers/staging/wilc1000/host_interface.c:1757 Handle_Key() warn:
returning -1 instead of -ENOMEM is sloppy

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>