usb: pch_udc: Fixed issue which does not work with g_ether

This PCH_UDC driver does not work normally when "Ethernet gadget" is used.

This patch fixed this issue.
The following was modified.
 - The FIFO flush process.
 - The descriptor creation process.
 - The adjustment of DMA buffer align.

Currently the PCH_UDC driver can work normally with "Ethernet gadget",
"Serial gadget" or "File-backed Storage Gadget".

Signed-off-by: Toshiharu Okada <toshiharu-linux@dsn.okisemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>