staging: wfx: check for memory allocation failures from wfx_alloc_hif

Currently calls to wfx_alloc_hif are not checking for a null return
when a memory allocation fails and this leads to null pointer
dereferencing issues.  Fix this by adding null pointer checks and
returning passing down -ENOMEM errors where necessary. The error
checking in the current driver is a bit sparse, so this may need
some extra attention later if required.

Fixes: f95a29d4

 ("staging: wfx: add HIF commands helpers")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20191221001543.15255-1-colin.king@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>