storage: flash_map: fix incomplete type

A pointer is not properly declared properly which causes the
build to fail.

flash_map_shell.c and flash_map.h do not include device.h so
the device struct has an incomplete type hence the build error.
Including device.h resolves the problem.

Also fixes a Wformat warning when referencing fa_off which is a pointer.
Cast the pointer to an uint32_t.

Fixes #48722

Signed-off-by: Steven Slupsky <sslupsky@gmail.com>