net: ethernet: fs_enet: cosmetic cleanups

Due to the age of the driver and the slow recent activity on it, the code
has taken some layers of dust. Clean the main driver file up so that it
passes checkpatch and also conforms with the net coding style.

Changes include :
 - Re-ordering of the variable declarations for RCT
 - Fixing the comment styles to either one-line comments, or net-style
   comments
 - Adding braces around single-statement 'else' clauses
 - Aligning function/macro parameters on the opening parenthesis
 - Simplifying checks for NULL pointers
 - Splitting cascaded assignments into individual assignments
 - Fixing some typos
 - Fixing whitespace issues

This is a cosmetic change and doesn't introduce any change in behaviour.
Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>