firmware(9): extend firmware_get() by a "no warn" flag.

With the upcoming usage from LinuxKPI but also from drivers
ported natively we are seeing more probing of various
firmware (names).

Add the ability to firmware(9) to silence the
"firmware image loading/registering errors" by adding a new
firmware_get_flags() functions extending firmware_get() and
taking a flags argument as firmware_put() already does.

Requested-by: zeising (for future LinuxKPI/DRM)
Sponsored-by: The FreeBSD Foundation
Sponsored-by: Rubicon Communications, LLC ("Netgate")
MFC after: 3 days
Reviewed-by: markj
Differential Revision: https://reviews.freebsd.org/D27413