usb: ehci: mxs: Use regulator_set_enable_if_allowed()

Since commit 4fcba5d5 ("regulator: implement basic reference
counter") the return value of regulator_set_enable() may be EALREADY or
EBUSY for fixed/GPIO regulators.

Switch to using the more relaxed regulator_set_enable_if_allowed() to
continue if regulator already was enabled or disabled.

This fixes the following error when running the 'ums' command:

=> ums 0 mmc 0
UMS: LUN 0, dev mmc 0, hwpart 0, sector 0x0, count 0xece000
Error enabling VBUS supply
g_dnl_register: failed!, error: -114
g_dnl_register failed

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>