devtools: update git headline prefix check for drivers

devtools: update git headline prefix check for drivers

For the patches that touch multiple drivers in same driver group,
script forces headline prefix start with drv group.

Like for net/a net/b net/c, patch title should be "net: x y z"

Update rule to let "driver" prefix in headline,
for above sample patch title becomes: "drivers/net: x y z"

This prevents patch confused with library with same name.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>