mtd: gpmi: Remove noisy error message

mx28evk board has a socket for NAND flash that comes with no NAND flash
populated, and then we get this message on every boot:

[    1.657603] gpmi-nand 8000c000.gpmi-nand: driver registration failed: -19

which is not very helpful, so get rid of this error message.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>