regmap: debugfs: Fix return from regmap_debugfs_get_dump_start

regmap_debugfs_get_dump_start should return the offset of the register
it should start reading from, However in the current code at one point
the code does not return correct register offset.

With this patch all the returns from this function takes reg_stride in
to consideration to return correct offset.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>