package/dahdi-linux: bump to 5c840cf43838e0690873e73409491c392333b3b8

- Fix build with kernel >= 5.4 thanks to
  https://git.asterisk.org/gitweb/?p=dahdi/linux.git;a=commit;h=45ac6a30f922f4eef54c0120c2a537794b20cf5c
- Fix build with kernel >= 5.6 thanks to
  https://git.asterisk.org/gitweb/?p=dahdi/linux.git;a=commit;h=34b9c77c9ab2794d4e912461e4c1080c4b1f6184
- Fix typo in dahdi-linux.mk
- Remove deprecated note about kernel >= 4.0 in Config.in
- Add patch to fix build failure with 32-bits kernels raised since
  https://git.asterisk.org/gitweb/?p=dahdi/linux.git;a=commit;h=ffcd08205c71dcb0e060836359418bef20f07ffa
- Update indentation in hash file (two spaces)

Fixes:
 - https://bugs.buildroot.org/show_bug.cgi?id=14311



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>