mtd: omap2: use nand_base defaults for polled I/O

The omap_{read,write}_buf{8,16}() functions are identical to the default
nand_base versions. Just let nand_base assign them in the
NAND_OMAP_POLLED case.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Tested-by: Pekon Gupta <pekon@ti.com>