iwlwifi: pcie: clean iwl_pcie_[rt]xq_inc_wr_ptr a bit

The various code blocks in iwl_pcie_[rt]xq_inc_wr_ptr
finally do the same things, so just merge them
all and make the functions cleaner.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>