igc: set the default return value to -IGC_ERR_NVM in igc_write_nvm_srwr

[ Upstream commit ebc8d125 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>