[ARM] vfp: fix leak of VFP_NAN_FLAG into FPSCR

The VFP code can leak VFP_NAN_FLAG into the FPSCR. It doesn't correspond
to any real FPSCR bit (and overlaps one of the exception flags).

Bug report from Daniel Jacobowitz

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>