hpt366: cleanup 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 manipulate the nIEN bit
in this method; therefore, just do nothing for the drives not on the quirk list.
Move the code to the left by using an early return and the 'else if' construct,
while at it....

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
[bart: fix checkpatch.pl warning]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>