powerpc/5200: Don't specify IRQF_SHARED in PSC UART driver

The MPC5200 PSC device is wired up to a dedicated interrupt line
which is never shared.  This patch removes the IRQF_SHARED flag
from the request_irq() call which eliminates the "IRQF_DISABLED
is not guaranteed on shared IRQs" warning message from the console
output.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>