net: stmmac: improve ndev->max_mtu setup readability

net: stmmac: improve ndev->max_mtu setup readability

Improve the readibility of the code setting ndev->max_mtu. This depends
on the hardware specific maximum defined by the MAC core, and also a
platform provided maximum.

The code was originally checking that the platform specific maximum was
between ndev->min_mtu..MAC core maximum before reducing ndev->max_mtu,
otherwise if the platform specific maximum was less than ndev->min_mtu,
issuing a warning.

Re-order the code to handle the case where the platform specific max is
below ndev->min_mtu, which then means that the subsequent test is
simply reducing ndev->max_mtu.

Update the comment, and add a few blank lines to separate the blocks of
code.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vImWA-0000000DrIl-1HZY@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>