ser2net: Fix compilation failures due to missing TIOCSRS485 macro

ser2net: Fix compilation failures due to missing TIOCSRS485 macro

Apply a patch sent upstream to fix a compilation failure present on some
architectures which do not define the TIOCSRS485 macro and do not
include the <asm-generic/ioctls.h> header. This is how the error looks
like:

devcfg.c:781:34: error: 'TIOCSRS485' undeclared (first use in this
function)

Patch sent upstream:
	https://sourceforge.net/p/ser2net/mailman/message/32905302/

Fixes:
	http://autobuild.buildroot.net/results/5376a3f64c95f14449b956df006741ffeca1350b/
	http://autobuild.buildroot.net/results/d182d94562a549c79c0c9fb01777a7cb83d7ab4c/
	http://autobuild.buildroot.net/results/478c5ad2f5541adef4218ad485aab4d78cf29210/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>