e1000e: remove no longer used e1000e_read_nvm_spi

This function is no longer used now that 82573 uses the eerd
read method as well. Thanks to Adrian Bunk for pointing this out.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>