platform/x86: wmi: use !p to check for NULL

Other parts of the code use the `!p` idiom to check
for NULL pointers, convert `find_guid_context()` to
do the same.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Link: https://lore.kernel.org/r/20210904175450.156801-19-pobrn@protonmail.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>