OMAP/serial: Fix misnamed variable

Fix misnamed variable to eliminate confusion.

Signed-off-by: Alexey Pelykh <alexey.pelykh@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>