urtwn: add an option to compile the driver without firmware specific code

• Add URTWN_WITHOUT_UCODE option (will disable any firmware specific code

when set).

• Do not exclude the driver from build when MK_SOURCELESS_UCODE is set

(URTWN_WITHOUT_UCODE will be enforced unconditionally).

• Do not abort initialization when firmware cannot be loaded;

behave like the URTWN_WITHOUT_UCODE option was set.

• Drop some unused variables from urtwn_softc structure.

Tested with RTL8188EU and RTL8188CUS in HOSTAP and STA modes.

Reviewed by: kevlo
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D4849