usb: dwc2: Update partial power down entering by system suspend

usb: dwc2: Update partial power down entering by system suspend

With current implementation the port power is being disabled,
which is not required by the programming guide. Also, if there
is a system which works only in "DWC2_POWER_DOWN_PARAM_NONE"
(clock gating) mode the current implementation does not set
Gate hclk bit in pcgctl register.

Rearranges and updates the implementation of entering to partial
power down power saving mode when PC is suspended to get
rid of many "if" statements and removes disabling of port power.

NOTE: Switch case statement is used for hibernation partial
power down and clock gating mode determination. In this patch
only Partial Power Down is implemented the Hibernation and
clock gating implementations are planned to be added.

Acked-by: Minas Harutyunyan <Minas.Harutyunyan@synopsys.com>
Signed-off-by: Artur Petrosyan <Arthur.Petrosyan@synopsys.com>
Link: https://lore.kernel.org/r/20210408094559.33541A022E@mailhost.synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>