brcmfmac: remove unused parameter of brcmf_sdcard_reg_read

The size parameter for brcmf_sdcard_reg_read is always 4. Remove it
to make the code neat.

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>