GCC4.6: Fix common/usb.c on xscale

The problem was that the code, when the function was compiled with -Os, was
misgenerated. As in the function description, this is likely another
manifestation of the bug in GCC.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Remy Bohmer <linux@bohmer.net>