Staging: vme: Allow size of 0 when disabling a window

The TSI148 driver currently does not allow a size of zero to be passed to a
window. Zero is a valid value if the window is being disabled. Allow
windows to be disabled and their registers cleared.

Signed-off-by: Martyn Welch <martyn.welch@gefanuc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>