net: stmmac: thead: ensure divisor gives proper rate

thead was checking that the stmmac_clk rate was a multiple of the
RGMII rates for 1G and 100M, but didn't check for 10M. Rather than
use this with hard-coded speeds, check that the calculated divisor
gives the required rate by multplying the transmit clock rate back
up to the stmmac clock rate and checking that it agrees.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Drew Fustini <drew@pdp7.com>
Link: https://patch.msgid.link/E1tlToD-004W3g-HB@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>