iwlwifi: pcie: make iwl_pcie_gen2_update_byte_tbl static

It is called within tx-gen2.c only.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>