misc: fs_loader: Fix alignment of fs_loader driver

DM_DRIVER_GET will redeclare the fs_loader driver without the correct
alignment. This causes GCC to use the default section alignment of 32
bytes. This in turn creates a gap in the linker list due to the padding
required to achieve the correct alignment, corrupting all further entries.
Use DM_DRIVER_REF instead, which doesn't redeclare anything.

Fixes: 0998a20c ("misc: fs_loader: Add function to get the chosen loader")
Signed-off-by: Sean Anderson <seanga2@gmail.com>