hw/pxa2xx.c: Fix handling of R/WC bits in PMCR

Fix a bug in handling the write-one-to-clear bits in the PMCR
which meant that we would always clear the bit even if the
value written was a zero. Spotted by Coverity (see bug 887883).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>