fs: exfat: Demote filesystem detection failure message to debug()

fs: exfat: Demote filesystem detection failure message to debug()

Demote "exFAT file system is not found" message to debug(). This is
printed when U-Boot attempts to auto-detect the filesystem via generic
filesystem API by attempting to mount the device, and fails to do so
because there is another filesystem in place. The libexfat-fuse code
prints this an error, which interferes with 'test_gpt' test. Demote
the message to debug().

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Marek Vasut <marex@denx.de>