sgiioc4: remove maskproc() method

Since the maskproc() method calls either mirror the interrupt en/disable via
the nIEN bit of the device control register done by the IDE core before issuing
a command or unmask the interrupt after a command executed in polled mode (when
interrupt is already not expected), it is pointless to implement this method
by manipulating the nIEN bit...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: jeremy@sgi.com
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>