PCI: correct pcie_set_readrq write size

When setting the PCI-E MRRS, pcie_set_readrq queries the current
settings via a pci_read_config_word call but writes the modified result
via a pci_write_config_dword.  This results in writing 16 more bits than
were queried.

Also, the function description comment is slightly incorrect.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>