fs: exfat: Inhibit "impossible" print on write to bogus file

fs: exfat: Inhibit "impossible" print on write to bogus file

Write into a bogus file, like '/.', triggers an "impossible"
print from the exfat core code. That should not be printed
in U-Boot, because U-Boot prints its own error message sooner.
Inhibit this error message.

The following command triggers the bogus print:
"
=> save host 0:0 1000008 /. 0x10
"

Fixes: b86a651b646c ("fs: exfat: Add U-Boot porting layer")
Signed-off-by: Marek Vasut <marex@denx.de>