miiphy: use strncpy() not sprintf()

In miiphy_register() the new device's name was initialised by passing a
string parameter as the format string to sprintf(). As this would cause
problems if it ever contained a '%' symbol, switch to using strncpy()
instead.

Signed-off-by: Laurence Withers <lwithers@guralp.com>
Cc: Andy Fleming <afleming@freescale.com>