brcm80211: fmac: remove brcmf_usb_attrib structure

Several fields in this structure are only written once or not used
at all. Remaining two fields have been moved and brcmf_usb_attrib
definition has been removed.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Kan Yan <kanyan@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>