mii: miiphy register address width change

This patch is added for PHY whose register offset value exceeds 0xFF and
cannot be used with "unsigned char" datatype in miiphy_read, miiphy_write
and miiphy_register functions. Datatype of register offset is changed to
unsigned short instead of unsigned char so that offset value greater then
0xFF can be used.

Signed-off-by: Chandan Nath <chandan.nath@ti.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>