test: Fix pointer overrun in dm_test_devm_regmap()

This tests calls regmap_read() which takes a uint pointer as an output
parameter. The test was passing a pointer to a u16 which resulted in an
overflow when the output was written. Fix this by following the
regmap_read() API and passing a uint pointer instead.

Signed-off-by: Andrew Scull <ascull@google.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Jean-Jacques Hiblot <jjhiblot@ti.com>
Cc: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>