iwlwifi: pcie: don't wake up the NIC when writing CSRs in MSIX mode

CSR registers are always available even when the NIC is not awake, no
need to wake up the NIC before accessing them. This has a huge impact
when we re-enable an interrupt at the end of the ISR since waking up the
NIC can take some time.

Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>