serial: max310x: use regmap methods for SPI batch operations

[ Upstream commit 285e76fc

 ]

The SPI batch read/write operations can be implemented as simple
regmap raw read and write, which will also try to do a gather
write just as it is done here.

Use the regmap raw read and write methods.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Link: https://lore.kernel.org/r/20220605144659.4169853-2-demonsingur@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: b35f8dbb

 ("serial: max310x: prevent infinite while() loop in port startup")
Signed-off-by: Sasha Levin <sashal@kernel.org>