Replace hardcoded constants by enumerated values in sis190_read_eeprom

The names of the enumerated values have been extracted from SiS'official
driver (v1.00.00 published on 2005/07/11).
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>