usb: musb-new: Cannot test unsigned member to be negative

usb: musb-new: Cannot test unsigned member to be negative

You cannot test an unsigned member of a struct for being negative, the
test will always fail. Instead assign the return value of
fdtdec_get_int, which returns an int, to a temporary variable declared
as an int, so that it can be tested for being negative before being
assigned to the unsigned struct member.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>