net/wifi-firmware-*: add support to set loader tunable

For drivers which currently need to set the
compat.linuxkpi.skb.mem_limit tunable add an option to the build
to create it automatically for each FLAVOR. There is no harm
if it is there multiple times setting it to the same value (apart
from file including and parsing overhead in loader -- however that
works).

Add the option to the rtw88 and rtw89 wifi-firmware Makefile.

Having it with the firmware package has multiple advantages:

• the user does not have to do any manual configuration anymore
• we do not have to configure setting it into the release builds
• bsdinstall does not need to know about it to set for the installed system (depending on driver).

Bump PORTREVISION.

Sponsored by: The FreeBSD Foundation
MFC after: 1 minute
X-MFC for: releng 14.2
Differential Revision: https://reviews.freebsd.org/D47481