bcma: export bcma_find_core_unit()

This function is used to get a specific core when there is more than
one core of that specific type. This is used in bgmac to reset all GMAC
cores.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>