driver: mtd: sf_ops: claim bus while doing memcpy

claim spi bus while doing memory copy, this will set up
the spi controller device control register before doing
a memory read.

Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Tested-by: Yebio Mesfin <ymesfin@ti.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>