mtd: chips: gen_probe: kill useless initializer in mtd_do_chip_probe()

The 'mtd' local variable is initialized but this value is never used,
thus kill that initializer.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>