net: dsa: mv88e6xxx: Use kcalloc()

Use kcalloc() instead of hand writing it. This is less verbose.

Also move the initialization of 'count' to save some LoC.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  18652	   5920	     64	  24636	   603c	drivers/net/dsa/mv88e6xxx/devlink.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  18498	   5920	     64	  24482	   5fa2	drivers/net/dsa/mv88e6xxx/devlink.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/2f4fca4ff84950da71e007c9169f18a0272476f3.1751200453.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jakub Kicinski <kuba@kernel.org>