USB: gadget: f_fs: Allocate extra buffer for IN endpoint

USB AXI prefetch requires additional 256 bytes than actual required
buffer size for USB IN endpoint. But Currently driver is allocating
for USB OUT endpoint. Hence fix this by allocating extra buffer for
USB IN endpoint only.

Change-Id: I3c2d299d8dce3d826b153856e2cd8366b33a2dd6
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>