spi: atmel-quadspi: Remove default mode setting at probe time

spi: atmel-quadspi: Remove default mode setting at probe time

The Serial Memory Mode (SMM) is enabled with atmel_qspi_set_cfg() on
each invocation of atmel_qspi_exec_op().  Setting SMM through
atmel_qspi_init() at probe time is redundant.

Removing the SMM setting at probe time should therefore 1) be safe to do
and 2) allows for setting it to a different value in a future
implementation of .xfer() which needs to disable SMM.

Signed-off-by: Alexander Dahl <ada@thorsis.com>