gpio: syscon: reduce message level when direction reg offset not in dt

gpio: syscon: reduce message level when direction reg offset not in dt

Now GPIO syscon driver produces bunch of warnings during the
boot of Kesytone 2 SoCs:
 gpio-syscon soc:keystone_dsp_gpio@02620240: can't read the dir register offset!
 gpio-syscon soc:keystone_dsp_gpio@2620244: can't read the dir register offset!

This message unintentionally was added using dev_err(), but its
actual log level is debug, because third cell of "ti,syscon-dev" is
optional.

Hence change it to dev_dbg() as it should be.

This patch fixes commit:
 5a3e3f8 ("gpio: syscon: retriave syscon node and regs offsets from dt")

Reported-by: Russell King <[email protected]>
Tested-by: Murali Karicheri <[email protected]>
Acked-by: Santosh Shilimkar <[email protected]>
Signed-off-by: Grygorii Strashko <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>