ULONG_MAX is used by a few drivers to figure out the highest available
clock rate via clk_round_rate(clk, ULONG_MAX). Since abs() takes a
signed value as input, the current logic effectively calculates with
ULONG_MAX = -1, which results in the worst parent clock being chosen
instead of the best one.

For example on Rockchip RK3588 the eMMC driver tries to figure out
the highest available clock rate. There are three parent clocks
available resulting in the following rate diffs with the existing
logic:

GPLL:   abs(18446744073709551615 - 1188000000) = 1188000001
CPLL:   abs(18446744073709551615 - 1500000000) = 1500000001
XIN24M: abs(18446744073709551615 -   24000000) =   24000001

As a result the clock framework will promote a maximum supported
clock rate of 24 MHz, even though 1.5GHz are possible. With the
updated logic any casting between signed and unsigned is avoided
and the numbers look like this instead:

GPLL:   18446744073709551615 - 1188000000 = 18446744072521551615
CPLL:   18446744073709551615 - 1500000000 = 18446744072209551615
XIN24M: 18446744073709551615 -   24000000 = 18446744073685551615

As a result the parent with the highest acceptable rate is chosen
instead of the parent clock with the lowest one.

Cc: stable@vger.kernel.org
Fixes: 49502408 ("mmc: sdhci-of-dwcmshc: properly determine max clock on Rockchip")
Tested-by: Christopher Obbard <chris.obbard@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20230526171057.66876-2-sebastian.reichel@collabora.comReviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>