net: dsa: mt7530: move enabling disabling core clock to mt7530_pll_setup()

Split the code that enables and disables TRGMII clocks and core clock.
Move enabling and disabling core clock to mt7530_pll_setup() as it's
supposed to be run there.

Add 20 ms delay before enabling the core clock as seen on the U-Boot
MediaTek ethernet driver.

Change the comment for enabling and disabling TRGMII clocks as the code
seems to affect both TXC and RXC.

Tested rgmii and trgmii modes of port 6 and rgmii mode of port 5 on MCM
MT7530 on MT7621AT Unielec U7621-06 and standalone MT7530 on MT7623NI
Bananapi BPI-R2.

Fixes: b8f126a8

 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Link: https://source.denx.de/u-boot/u-boot/-/blob/29a48bf9ccba45a5e560bb564bbe76e42629325f/drivers/net/mtk_eth.c#L589
Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Link: https://lore.kernel.org/r/20230320190520.124513-1-arinc.unal@arinc9.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>