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

Fix an error in commit afd4a652 which meant that writing a zero
to the RW bits in the PMCR wouldn't actually clear them. (Error
spotted by Andrzej Zaborowski.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>