ieee802154: atusb: fix firmware version check to enable frame retries

Geert reported: as fw_ver_maj is unsigned char, gcc 4.1.2 complains:

warning: comparison is always true due to limited range of data type

Besides the warning the old check would also fail for firmware versions
like 1.x with x < 3. These would support frame retries, but the driver
would not enable the feature.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>