drivers: usb: stm32: Dont disable ULPI clock on low power

When running the cdc_acm demo on a board with an external ULPI
phy, the device is unable to enumerate:

usb 1-1.8.3.1: new full-speed USB device number 51 using ehci-pci
usb 1-1.8.3.1: device descriptor read/64, error -32
usb 1-1.8.3.1: device descriptor read/64, error -32

the console shows:

[00:00:00.001,000] cdc_acm_echo: Wait for DTR
[00:00:00.007,000] usb_cdc_acm: Device suspended
[00:00:00.902,000] usb_cdc_acm: Device resumed
[00:00:00.902,000] usb_cdc_acm: from suspend
[00:00:25.526,000] usb_cdc_acm: Device suspended

By not disabling the ULPI clock in low power, the usb enumeration
is working and we can run the cdc_acm demo. While touching
this code, add some comments to clarify the macro nesting.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>