backlight: lm3630a: Bump REG_MAX value to 0x50 instead of 0x1F

In the lm3630a_chip_init we try to write to 0x50 register, which is
higher value then the max_register value, this resulted in regmap_write
return -EIO.

Fix this by bumping REG_MAX value to 0x50.

This code was introduced with the chip revision in commit 28e64a68,
however setting filter strength was failing silently because it used
unsigned int for storing and comparing the return values. Bug related to
signedness was fixed in 2a0c316b, which made it error out correctly
instead of failing silently.

I found this issue by using this driver on LGE Nexus 5 (hammerhead).
After this commit lm3630a_chip_init succeeds instead of failing with
-EIO.

Fixes: 28e64a68 ("backlight: lm3630: apply chip revision")
Fixes: 2a0c316b ("drivers/video/backlight/lm3630a_bl.c: fix
signedness bug in lm3630a_chip_init()")

Suggested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Bhushan Shah <bshah@kde.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>