ixgbe: don't do arithmetic operations on bitmasks

Make the calculation of eerd consistent between the read and write functions
by using | instead of + for IXGBE_EEPROM_RW_REG_START

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>