loader: beri_sdcard_disk_print() needs to return int.

The https://reviews.freebsd.org/rS308434 did change the return type for
dv_print callbacks, but the return type for beri_sdcard_disk_print()
was left unchanged, causing compile errors.

Reported by: cy
Reviewed by: brooks, rstone, allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D8515