mtd: devices: elm: configure parallel channels based on ecc_steps

ELM hardware can process up to maximum of 8 hannels in parallel for
ECC error detection. Currently the number of channels getting configured for
processing is static determined by macro ERROR_VECTOR_MAX. However, the actual
number of channels that need to be processed is the ECC step number.
This patch just avoids configuring extra unused channels.

Signed-off-by: Pekon Gupta <pekon@ti.com>
Reviewed-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>