Since we don't support DM in SPL, undefine the DM USB in SPL build,
so it can use non-DM USB driver.
Signed-off-by: Ye Li
(cherry picked from commit 1e02825ab852f31111b875af9b84f82a974df64c)
(cherry picked from commit 62efea1e99a02fc282d0a22c36fddb1b14a09612)