usb: Change transfer_len type to int32_t in usb_request_handler typedef

The functions that implement usb_request_handlers are already using an
int32_t for transfer_len, so lets make the typedef match.  This address
a potential issue in the future when the typedef of int32_t changes (or
when building with newlib).

Change-Id: I6e478551c38f2040b0dcec47c2e4c565c27acdd0
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>