brcmfmac: combine bcmsdh source files into one

brcmfmac: combine bcmsdh source files into one

The driver has four files specific to SDIO host interface handling.
This commit reduces that by merging two closely related source files.

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