net: smc911x: Convert IO accessors to {read,write}{w,l}()

Convert the IO accessors to standard ones instead of using volatile
void pointers, as those do not cover all the bus access details.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>