if_muge: retire lan78xx_eeprom_read

lan78xx_eeprom_read just checked for EEPROM presence then called
lan78xx_eeprom_read_raw if present, and had only one caller. Introduce
lan78xx_eeprom_present to check for EEPROM presence, and use it in the
one place it is needed.

This is used by r334964, which was accidentally committed out-of-order
from my work tree.

Reported by: markj
Sponsored by: The FreeBSD Foundation