[ARM] 4993/1: <IMX UART>: Trivial: Remove unused defines

This patch removes the unused defines NR_PORTS and IMX_ISR_PASS_LIMIT in
the imx serial driver.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>