spi: spi-mem: Claim SPI bus before spi mem access

It is necessary to call spi_claim_bus() before starting any SPI
transactions and this restriction would also apply when calling spi-mem
operations. Therefore claim and release bus before requesting transfer
via exec_op.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Tested-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Tested-by: Stefan Roese <sr@denx.de>
Tested-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> #zynq-microzed