usb: host: ehci-generic: Fix error check

Commit 81755b8c ("usb: host: ehci-generic: Make resets and clocks
optional") improved the error check to cover the reset property being
optional. However this was using the wrong error variable for the
check, so would now never fail.

Use the correct error variable for checking the result of
reset_get_bulk(), to actually report genuine errors.

Fixes: 81755b8c ("usb: host: ehci-generic: Make resets and clocks optional")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>