net: e1000: do not attempt to set hwaddr for i210 without FLASH

commit f1bcad22 ("net: e1000: add support for writing to EEPROM")
adds support for storing hwaddr in EEPROM however i210 devices do not
support this and thus results in errors such as:
Warning: e1000#0 failed to set MAC address'

Check if a flash device is present and if not return -ENOSYS indicating
this is not supported.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>