debian/{compat,control}: bump debhelper compatibility to 11

This silences the following lintian notice:

P: libam7xxx source: package-uses-old-debhelper-compat-version 9
N:
N:    The debhelper compatibility version used by this package is marked as
N:    not recommended by the debhelper developer. You may consider using a
N:    recommended compatibility version.
N:
N:    The compatibility version can be set in (preferred) debian/compat or by
N:    setting and exporting DH_COMPAT in debian/rules. If it is not set in
N:    either place, debhelper defaults to the deprecated compatibility version
N:    1.
N:
N:    Refer to the debhelper(7) manual page for details.
N:
N:    Severity: pedantic, Certainty: certain
N:
N:    Check: debhelper, Type: source
N: