drivers/spi_nor: de-pessimize reads

SPI NOR devices require that writes be performed within only one page at
a time.  There is no such limitation on reads.  Remove the code that
forced reads to be performed in 256-byte chunks.

Backport: 2.0 d60d16d4

Signed-off-by: Peter A. Bigot <pab@pabigot.com>