net: am2150: fix nmclan_cs.c shared interrupt handling

A recent patch tried to work around a valid warning for the use of a
deprecated interface by blindly changing from the old
pcmcia_request_exclusive_irq() interface to pcmcia_request_irq().

This driver has an interrupt handler that is not currently aware
of shared interrupts, but can be easily converted to be.
At the moment, the driver reads the interrupt status register
repeatedly until it contains only zeroes in the interesting bits,
and handles each bit individually.

This patch adds the missing part of returning IRQ_NONE in case none
of the bits are set to start with, so we can move on to the next
interrupt source.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 5f5316fcd08ef7 ("am2150: Update nmclan_cs.c to use update PCMCIA API")
Signed-off-by: David S. Miller <davem@davemloft.net>