Patch from Andrew Victor

The serial_core already manages the power state of the UARTs, and
therefore it shouldn't suspend a UART which was previously suspended.

This patch modifies serial_core only call the UART-specific
power-management function if the PM state is actually changing.
Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>