ata: ahci_imx: warn when disabling ahci link

When the AHCI link is disabled, it can't be re-enabled except by
resetting the entire SoC.  Rather than doing this silently print
some kernel messages to inform the user, along with how to avoid
this.

tj: Put a long printf format string on a single line.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tejun Heo <tj@kernel.org>