mwifiex: pcie: de-duplicate buffer allocation code

This code was duplicated as part of the PCIe FLR code added to this
driver. Let's de-duplicate it to:

 * make things easier to read (mwifiex_pcie_free_buffers() now has a
   corresponding mwifiex_pcie_alloc_buffers())
 * reduce likelihood of bugs
 * make error logging equally verbose
 * save lines of code!

Also drop some of the commentary that isn't really needed.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>