net: stmmac: fix order of dwmac5 FlexPPS parametrization sequence

net: stmmac: fix order of dwmac5 FlexPPS parametrization sequence

So far changing the period by just setting new period values while
running did not work.

The order as indicated by the publicly available reference manual of the i.MX8MP [1]
indicates a sequence:

 * initiate the programming sequence
 * set the values for PPS period and start time
 * start the pulse train generation.

This is currently not used in dwmac5_flex_pps_config(), which instead does:

 * initiate the programming sequence and immediately start the pulse train generation
 * set the values for PPS period and start time

This caused the period values written not to take effect until the FlexPPS output was
disabled and re-enabled again.

This patch fix the order and allows the period to be set immediately.

[1] https://www.nxp.com/webapp/Download?colCode=IMX8MPRM

Fixes: 9a8a02c9d46d ("net: stmmac: Add Flexible PPS support")
Signed-off-by: Johannes Zink <j.zink@pengutronix.de>
Link: https://lore.kernel.org/r/20230210143937.3427483-1-j.zink@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>