drivers: udc: stm32: fix USB_OTG_HS_EMB_PHY macro value check

USB_OTG_HS_EMB_PHY macro is always defined, so
`defined(USB_OTG_HS_EMB_PHY)` always results in true. This means that
driver always selected `USB_OTG_SPEED_HIGH_IN_FULL` and never
`USB_OTG_SPEED_HIGH`.

Fix that by checking value of defined macro.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>