backlight: qcom-wled: Rename PM8941* to WLED3

Rename the PM8941* references as WLED3 to make the driver
generic and have WLED support for other PMICs. Also rename
"i_boost_limit" and "i_limit" variables to "boost_i_limit"
and "string_i_limit" respectively to resemble the corresponding
register names.

Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>