[ARM] 3748/3: Correct error check in vfp_raise_exceptions

Patch from Daniel Jacobowitz

The recent fix to hide VFP_NAN_FLAG broke the check in vfp_raise_exceptions;
it would attempt to deliver an exception mask of 0xfffffeff instead of reporting
a serious error condition using printk.  Define a safe constant to use for
an invalid exception maskm, and use it at both ends.

Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>