gpio: qpnp-pin: Fix compilation error on enabling the debug option

Fix the incorrect mask/shift values specified for 'dtest-sel'
debugfs entry.

Also, add a special handling to enable/disable dtest-sel via
debugfs.
echo 0 > /d/qpnp_pin/pm8xxx_gpio/dtest-sel to disable DTEST
echo 1...4 > /d/qpnp_pin/pm8xxx_gpio/dtest-sel to enable and select DTEST

CRs-Fixed: 970023
Change-Id: I51e9af5d71bbf8b9b9023f6f3d747550e0fab0fa
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>