boot_serial: fix misuse of 'matched' param from zcbor_map_decode_bulk()

The 'matched' param in zcbor_map_decode_bulk() function is 'pointer to
the counter of matched keys', not length of payload buffer.

Fixes: fac2cabe ("boot_serial: Add image state set/get")
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
(cherry picked from commit f2cb5502)