test: fix test/dm/regmap.c

regmap_read() only fills the first two bytes of val. The last two bytes are
random data from the stack. This means the test will fail randomly.

For low endian systems we could simply initialize val to 0 and get correct
results. But tests should not depend on endianness. So let's use a pointer
conversion instead.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>