xtensa: don't limit csum_partial export by CONFIG_NET

csum_partial and csum_partial_copy_generic are defined unconditionally
and are available even when CONFIG_NET is disabled. They are used not
only by the network drivers, but also by scsi and media.
Don't limit these functions export by CONFIG_NET.

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>