i.MX31: fixed CTRL-C detection

The Register URXD contains status information in bits [15..8].
With status bit 15 set, CTRL-C was reported as 0x8003 instead
of 0x03. Therefore CTRL-C was not detected.
To solve this, bits [15..8] were masked out now.

Signed-off-by: Juergen Kilb <J.Kilb@gmx.de>
Acked-by: Felix Radensky <felix@embedded-sol.com>