net: e1000: Mark _disable_wr() and _write_status() as __maybe_unused

Per the comments, e1000_spi_eeprom_disable_wr() and
e1000_spi_eeprom_write_status() have been tested.
Remove the #if 0, #endif and mark them as __maybe_unused.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>