nand_spl: Fix large page nand_command()

This patch changes the large page nand_command() routine to use a word
offset instead of a byte offset. The 'offs' argument gets divided by 2
so that the offset passed to nand_command() is still by byte offset.
Originally, the offset was not shifted and when too high an offset was
requested the nand chip would attempt to read non-existent data.

Signed-off-by: Alex Waterman <awaterman@dawning.com>