usb_bam: Allocate 512 bytes more than actual data fifo size

On some platforms which uses USB 2.0 Chip idea controller, AXI
prefetch feature(APF) is added to support higher data rates. This
APF feature requires additional 128 bytes alloactated at the end of
circular buffer which is not used. Hence allocate additional 512
bytes than required data fifo size as per hardware programming guide.

Change-Id: I9d45d4aabfd3b163ffd2cd05939c8c2db7adfdd9
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>