spi: spi-mem: Constify spi_mem->name

There is no reason to make spi_mem->name modifiable. Moreover,
spi_mem_ops->get_name() returns a const char *, which generates a gcc
warning when assigning the value returned by spi_mem_ops->get_name()
to spi_mem->name.

Fixes: 5d27a9c8 ("spi: spi-mem: Extend the SPI mem interface to set a custom memory name")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>