brcmsmac: remove ai_findcore()

bcma also stores a pointer to the chipcommon core in its struct,
brcmsmac should use it and not search for the core by its own.

Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>