staging: brcm80211: removed global variable from sdio fullmac

Code cleanup. bus->card is assigned in brcmf_sdbrcm_probe (before
brcmf_sdbrcm_probe_attach()). Since w_sdreg32() and r_sdreg32() are called
only after that assignment, they can safely use bus->card. Thus there
is no instance left where brcmf_sdcard_reg_read() or brcmf_sdcard_reg_write()
is called with a NULL parameter, so the mechanism in bcmsdh.c that deals with
a NULL pointer could be deleted.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>