net: renesas: rswitch: use FIELD_PREP for remaining MPIC register fields

Commit fb9e6039 ("net: renesas: rswitch: fix initial MPIC register
setting") converted setting some MPIC fields to FIELD_PREP.

To keep common style, do the same with mii bus related fields of the
same register.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://patch.msgid.link/20241216071957.2587354-3-nikita.yoush@cogentembedded.com


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