net: stmmac: stmmac_is_jumbo_frm() len should be unsigned

net: stmmac: stmmac_is_jumbo_frm() len should be unsigned

stmmac_is_jumbo_frm() and the is_jumbo_frm() methods take skb->len
which is an unsigned int. Avoid an implicit cast to "int" via the
method parameter and then incorrectly doing signed comparisons on
this unsigned value.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/E1vLIWR-0000000Ewkf-1Tdx@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>