urtwn(4): rework ROM reading.

• Add error handling for urtwn_(r88e_)read_rom() and

urtwn_efuse_*() functions.

• Remove code duplication between urtwn_efuse_read() and

urtwn_r88e_read_rom().

• Merge r88e_rom and (r92c_)rom structures

(only one of them can be used at the same time).

• Other minor fixes / improvements.

Tested with RTL8188EU, STA mode
(URTWN_DEBUG + USB_DEBUG, hw.usb.urtwn.debug=3, no visual differences).

Reviewed by: kevlo
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D4253