spl: ymodem: Move GZ handling out of YModem session

In case the gunzip() call fails, it will print an error message.
If that happens within the YModem session, the error message will
not be displayed and would be useless. Move the gunzip() call out
of the YModem session to make those possible error messages visible.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Tom Rini <trini@konsulko.com>