[SERIAL] Fix matching of MMIO ports

The function uart_match_port() incorrectly compares the ioremap'd
virtual addresses of ports instead of the physical address to find
duplicate ports for MMIO based UARTs. This fixes it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>