phy: rockchip-samsung-dcphy: fix out-of-range max_register

The PHY register block is 64KB, so with a register stride of 4 the
last accessible register sits at offset 0xfffc. max_register names
0x10000, one register past the end of the mapping: dumping the
registers through the regmap debugfs interface reads beyond the
ioremapped region and oopses on the unmapped page. The oops fires
with the regmap lock held, so later PHY operations deadlock.

Fixes: b2a1a2ae7818 ("phy: rockchip: Add Samsung MIPI D-/C-PHY driver")
Cc: stable@vger.kernel.org
Signed-off-by: Jason Yang <jason98166@gmail.com>
Assisted-by: Claude:claude-opus-5
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patch.msgid.link/20260811-dcphy-maxreg-v1-v1-1-aa63f6a63a64@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>