net: dsa: microchip: Fix LAN937X set_ageing_time function

net: dsa: microchip: Fix LAN937X set_ageing_time function

The aging count is not a simple 20-bit value but comprises a 3-bit
multiplier and a 20-bit second time.  The code tries to use the
original multiplier which is 4 as the second count is still 300 seconds
by default.

As the 20-bit number is now too large for practical use there is an option
to interpret it as microseconds instead of seconds.

Fixes: 2c119d9982b1 ("net: dsa: microchip: add the support for set_ageing_time")
Signed-off-by: Tristram Ha <tristram.ha@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241218020224.70590-3-Tristram.Ha@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>