staging: wilc1000: check for kmalloc allocation failures

There are three kmalloc allocations that are not null checked which
potentially could lead to null pointer dereference issues. Fix this
by adding null pointer return checks.

Detected by CoverityScan, CID#1466025-27 ("Dereference null return")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>