usb: host: ehci-generic: Remove DM_REGULATOR flag

Since commit 16cc5ad0 ("power: regulator: add dummy helper")
regulator dummy helper are always available even if DM_REGULATOR
is not set.
DM_REGULATOR flag is no more needed to protect no DM core,
remove it.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>