fbdev: mmp: Fix deferred clk handling in mmphw_probe()

When dev_err_probe() is called, 'ret' holds the value of the previous
successful devm_request_irq() call.
'ret' should be assigned with a meaningful value before being used in
dev_err_probe().

While at it, use and return "PTR_ERR(ctrl->clk)" instead of a hard-coded
"-ENOENT" so that -EPROBE_DEFER is handled and propagated correctly.

Fixes: 81b63420 ("fbdev: mmp: Make use of the helper function dev_err_probe()")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Helge Deller <deller@gmx.de>