usb: gadget: Use 512 bytes as min RX buffer size

USB ep max packet size varies by connection speed. For example,
bulk ep max packet size(in bytes) for full speed is 64, high
speed is 512 and super speed is 1024. RX buffer is always
rounded up to be multiples of ep max packet size. Data transfer
failure reported when RX buffer is less than 128 bytes (example:
64 bytes). Failure includes unupdated buffer which results in
pplication level issues such as reset/ep halt. Workaround these
issues by making sure RX buffer size is minimum 512 bytes.

Change-Id: I6bcf7195dce45567f1696c2ae42bdf6d3fbdb00a
Signed-off-by: Mayank Rana <mrana@codeaurora.org>