staging: wfx: check pointers returned by allocations

Until now, the driver did not always check if the allocations success.

The issue was discussed here:
   https://lore.kernel.org/netdev/2026476.QLiXXEGFCf@pc-42/



Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200515083325.378539-4-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>