LinuxKPI: firmware, implement deferred loading for "nowait"

Change linuxkpi_request_firmware_nowait() to deferred firmware loading
scheduling a task. This changes behaviour in some cases that we
return from loading the driver before the driver is finished
initialising if the driver does not deal with it (wait).
This brings the behaviour one would expect from when this function is
called and I implemented it to see if it would help a specific case.

Sponsored by: The FreeBSD Foundation
MFC after: 12 days
Reviewed by: hselasky, imp (earlier version)
Differential Revision: https://reviews.freebsd.org/D30830