tty: serial: omap: switch over to devm_ioremap_resource

just using helper function to remove some duplicated
code a bit. While at that, also move allocation of
struct uart_omap_port higher in the code so that
we return much earlier in case of no memory.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>