[SCSI] sgiwd93: Fix compilation warning

The remove() callback in platform drivers should return int in
accordance to the definition of the platform_driver structure.
However, the SGI-specific WD93 SCSI controller driver defines
the callback as a void function, which causes the following
compilation warning:

drivers/scsi/sgiwd93.c:314: warning: initialization from
incompatible pointer type

This patch fixes the warning by changing the return type of
the remove() callback to what the core driver code requires.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>