b44: remove __exit from b44_pci_exit()

WARNING: drivers/net/ethernet/broadcom/built-in.o(.init.text+0x5d): Section mismatch in reference from the function b44_init() to the function .exit.text:b44_pci_exit()

module exits with b44_cleanup()

Signed-off-by: Nikola Pajkovsky <n.pajkovsky@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>