This patch sets the default return value to -IGC_ERR_NVM in
igc_write_nvm_srwr. Without this change it wouldn't lead to a shadow RAM
write EEWR timeout.

Fixes: ab405612 ("igc: Add NVM support")
Signed-off-by: Kevin Lo <kevlo@kevlo.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>