staging: et131x: Use for loop to initialise contiguous macstat registers to zero

Replace a long list of contiguous writel() calls with a for loop iterating
over the same address values.

Also remove redundant comments on the macstat registers, the variable names
are good enough.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>