OMAP: UART: don't resume UARTs that are not enabled.

Add additional check to omap_uart_resume_idle() so that only
enabled (specifically, idle-enabled) UARTs are allowed to resume.
This matches the existing check in prepare idle.

Without this patch, the system will hang if a board is
configured to register only some uarts instead of all of
them and PM is enabled.

Cc: Govindraj R. <govindraj.raja@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
[tony@atomide.com: updated description]
Signed-off-by: Tony Lindgren <tony@atomide.com>