usb: Fix USB keyboard polling via control endpoint

USB keyboard polling failed for some keyboards on PowerPC 5020.
This was caused by requesting only 4 bytes of data from keyboards that
produce an 8 byte HID report.

Signed-off-by: Adrian Cox <adrian@humboldt.co.uk>
Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Marek Vasut <marex@denx.de>