usb: gadget: fix usb_ep_align_maybe endianness and new usb_ep_align

USB spec specifies wMaxPacketSize to be little endian (as other properties),
so when using this variable in the driver we should convert to the current
CPU endianness if necessary.

This patch also introduces usb_ep_align() which does always returns the
aligned buffer size for an endpoint. This is useful to be used by USB requests
allocator functions.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>