usb: dwc2: use dev_read_bool() instead of fdt_getprop()

Use dev_read_bool() instead of fdt_getprop() to get the property
from DTS. And add a comment for "hnp-srp-disable" property to
fully describe its effect.

Signed-off-by: Meng Dongyang <daniel.meng@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>