mtd: gpmi-nand: Improve logging style

Improve logging style by prefixing the pr_ messages with "gpmi_nand".

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>