mtd: rawnand: gpmi: Use nand_extract_bits()

Drop the use of gpmi_copy_bits() in favor of the NAND helper
nand_extract_bits().

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200508171805.8627-1-miquel.raynal@bootlin.com