brcm80211: smac: change ai_findcoreidx() to ai_findcore()

Instead of returning the core index the function now returns
the bcma device for the requested core id. This function is
now exposed in the header file.

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