nand: pxa3xx: Increase READ_ID buffer and make the size static

The read ID count should be made as large as the maximum READ_ID size,
so there's no need to have dynamic size. This commit sets the hardware
maximum read ID count, which should be more than enough on all cases.
Also, we get rid of the read_id_bytes, and use a macro instead.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>