[SCSI] ch: fix sparse shadowed variable warnings

Replace the global err array with ch_err.
drivers/scsi/ch.c:271:6: warning: symbol 'err' shadows an earlier one
drivers/scsi/ch.c:116:3: originally declared here

Replace the temporary cmd buffer with ch_err to avoid shadowing the cmd
function parameter.
drivers/scsi/ch.c:724:11: warning: symbol 'cmd' shadows an earlier one
drivers/scsi/ch.c:596:20: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>