drivers: mtd: m25p80: convert "bool" read check into an enum

This is a cleanup prior to adding quad read support. This will facilitate
easy addition of more read commands check under an enum rather that defining a
separate bool for it.

Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>