ide/atapi: Introduce CHECK_READY flag for commands

Some commands are supposed to report a Not Ready Condition (i.e. they require
a medium to be present in order to execute successfully). Instead of
duplicating the check in each command implementation, let's add a flag and
check it before calling the command.

This patch only converts existing checks, it does not introduce new checks for
any of the other commands that can/should report a Not Ready Condition.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
