net: stmmac: use first_desc for TBS

net: stmmac: use first_desc for TBS

Rather than freshly getting a pointer for the TBS descriptor (because
we want to access its enhanced fields) convert the existing first_desc
basic descriptor to a pointer to the enhanced descriptor.

Add a comment explaining why it is safe to convert from the basic
descriptor pointer to the enhanced descriptor pointer.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1w3d0W-0000000DfM7-2BMA@rmk-PC.armlinux.org.uk
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
[pabeni@redhat.com: fixed comment typo]
Signed-off-by: Paolo Abeni <pabeni@redhat.com>