serial: max310x: Use devm_clk_get_optional() to get the input clock

[ Upstream commit 974e454d6f96da0c0ab1b4115b92587dd9406f6a ]

Simplify the code which fetches the input clock by using
devm_clk_get_optional(). If no input clock is present
devm_clk_get_optional() will return NULL instead of an error
which matches the behavior of the old code.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20201007084635.594991-2-andy.shevchenko@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: 8afa6c6decea ("serial: max310x: fail probe if clock crystal is unstable")
Signed-off-by: Sasha Levin <sashal@kernel.org>