mtd: spi-nor: improve wait-till-ready timeout loop

There are a few small issues with the timeout loop in
spi_nor_wait_till_ready():

 * The first operation should not be a reschedule; we should check the
   status register at least once to see if we're complete!

 * We should check the status register one last time after declaring the
   deadline has passed, to prevent a premature timeout error (this is
   theoretically possible if we sleep for a long time after the previous
   status register check).

 * Add an error message, so it's obvious if we ever hit a timeout.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Huang Shijie <shijie.huang@intel.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>