brcm80211: smac: remove ai_switch_core() function

The function ai_switch_core() is no longer needed and its counterpart
ai_restore_core() as well, because interrupts disabling is not needed
anymore.

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>