net: dc2114x: Use standard I/O accessors

The current dc21x4x driver accesses its memory mapped registers directly
instead of using the standard I/O accessors. This can cause problems on
some systems as the accesses can get out of order. So convert the direct
volatile dereferences to use the normal in/out macros.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Ramon Fried <rfried.dev@gmail.com>