gpio/gpio-stmpe: Fix the value returned by _get_value routine

The present _get_value routine returns the contents of the GPIO Monitor Pin
Status Register(GPMR) starting from the bit whose value is requested to BIT 0
(irrelevant bits are replace by 0).

For e.g. if we request the value of GPIO 6 in the earlier implementation the
value returned is:

	BIT6 followed by 6 0's

whereas it should just return BIT6.

This patch addresses the same.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@st.com>
Reviewed-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>