spi: fsl_qspi: Fix qspi_op_rdsr memcpy issue

In current driver, we always copy 4 bytes to the dest memory.
Actually the dest memory may be shorter than 4 bytes.
Add an argument to indicate the dest memory length.
Avoid writing memory outside of the bounds.

Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>
Reviewed-by: York Sun <york.sun@nxp.com>