dfu:usb:fix: Read the "filesize" environment variable only when file read

The "filesize" environment variable shall be read only when relevant
file is read.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Marek Vasut <marex@denx.de>