v4l2: Fix H.264 level 3 string representation

The string_to_level function handles "3" so the level_to_string function should
do the same, to prevent caps negotiation issues.
8 jobs for v4l-hw in 31 minutes and 59 seconds (queued for 1 minute and 28 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #161165
gst indent

00:00:26

passed #161164
manifest

00:00:30

 
  Build
passed #161166
build fedora x86_64

00:05:18

 
  Test
passed #161167
check fedora

00:03:42

 
  Full Builds
passed #161168
build android arm64 api28

00:04:06

passed #161170
build cerbero cross win32

00:10:55

passed #161171
build cerbero cross win64

00:09:06

passed #161169
build cerbero fedora x86_64

00:06:59