staging/easycap: implement strerror function

Replace long switch statements that just print out errno
with strerror function. It reduces around 700 lines from the code.

The function should be probably dropped at all but leave for now
to not break currently expected debug output.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>