scsi: Correct types of scsi_read/write()

The block device expects to see lbaint_t for the blknr parameter. Change
the SCSI read/write functions to suit.

This fixes the following build warnings for coreboot:

cmd_scsi.c: In function ‘scsi_scan’:
cmd_scsi.c:119:30: error: assignment from incompatible pointer type [-Werror]
cmd_scsi.c:120:32: error: assignment from incompatible pointer type [-Werror]

Signed-off-by: Simon Glass <sjg@chromium.org>