stmmac: Fix typo in stmmac_pci.c

Correct spelling "regiser" to "register" in
drivers/net/ethernet/stmicro/stmmac/stmmac_pci.c

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>