mtd: nand: pxa3xx: Use waitfunc() to wait for the device to be ready

In pxa3xx_nand_sensing() instead of simply using info->is_ready
after issuing a command, the correct way of checking is to wait
for the device to be ready through the chip's waitfunc().

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>