hpet: place read-only bits directly in "new_val"

The variable "val" is used for two different purposes.  As an intermediate
value when writing configuration registers, and to store the cleared bits
when writing ISR.

Use "new_val" for the former, and rename the variable so that it is clearer
for the latter case.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>