backlight: qcom-spmi-wled: Expose max_avail_current for switch via sysfs

Currently, max_brightness for flash and torch devices in WLED
is calculated based on the full scale current (FSC) and number
of strings enabled. However, max_avail_current is determined
based on the battery parameters. Add a sysfs parameter for switch
device to show available maximum current. At present, this is
going to show the maximum brightness.

Change-Id: Ic0b5f5c52a13193e314b5d7f898128a81385200e
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>