if_smsc: set MII BUSY bit to read/write PHY regs

Per the datasheet, the BUSY bit must be set when reading or writing PHY
registers. From Linux commit 80928805babf.

Submitted by: Arshan Khanifar
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15217