bnxt_en: initialize rc to zero to avoid returning garbage

rc is not initialized so it can contain garbage if it is not
set by the call to bnxt_read_sfp_module_eeprom_info. Ensure
garbage is not returned by initializing rc to 0.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>