media: mantis: prevent staying forever in a loop at IRQ

As warned by smatch:
	drivers/media/pci/mantis/mantis_uart.c:105 mantis_uart_work() warn: this loop depends on readl() succeeding

If something goes wrong at readl(), the logic will stay there
inside an IRQ code forever. This is not the nicest thing to
do :-)

So, add a timeout there, preventing staying inside the IRQ
for more than 10ms.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>