backports: fix uninstall filename

Upstream commit: 1256d3ea1856aca94f82486714758c82c60f92fb

The uninstall script calculated the filename in the wrong way. It added
the current working dir into the file name, this resulted in names like
this:
/lib/modules/2.6.32-431.3.1.el6.x86_64/updates//home/hauke/backports-20140124/drivers/net/wireless/ipw2x00/ipw2100.ko
The correct one would be this
/lib/modules/2.6.32-431.3.1.el6.x86_64/updates/drivers/net/wireless/ipw2x00/ipw2100.ko

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>