ixgbe: cleanup checksum to allow error results

Currently the shared code checksum calculation function only
returns a u16 and cannot return an error code. Unfortunately
a variety of errors can happen that completely prevent the
calculation of a checksum. So, change the function return value
from a u16 to an s32 and return a negative value on error, or the
positive checksum value when there is no error.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>