brcm80211: fmac: remove drive strength code for unsupported chips

bcm4325 and bcm4336 are not supported by brcmfmac. Remove the
drive strength setting code specific for these chips.

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