smc91c92_cs: fix the problem that lan & modem does not work simultaneously

smc91c92_cs:
  Fix the problem that lan & modem does not work simultaneously
  in the Megahertz multi-function card.
  We need to write MEGAHERTZ_ISR to retrigger interrupt.

Signed-off-by: Ken Kawasaki <ken_kawasaki@spring.nifty.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>