usb-misc: sisusbvga: fix error path

Remove a call to dev_err() that was reporting an unsuccesful call to
kmalloc(), as reporting memory allocation failures is redundant. Instead
of logging the error, clean up previously allocated resources and abort
the probe with -ENOMEM. Before this change sisusb->SiS_Pr could be
dereferenced even if null after failure of memory allocation.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>