debian/control: drop the dependency on dpkg-dev

debian/control: drop the dependency on dpkg-dev

dpkg-dev is an essential package and it's not necessary to depend on it
unless a versioned dependency is used.

Removing the dependency fixes the following lintian error:

E: libam7xxx source: depends-on-build-essential-package-without-using-version dpkg-dev [build-depends: dpkg-dev]
N:
N:    The package declares a depends on a build essential package without
N:    using a versioned depends. Packages do not have to build-depend on any
N:    package included in build-essential. It is the responsibility of anyone
N:    building packages to have all build-essential packages installed. The
N:    only reason for an explicit dependency on a package included in
N:    build-essential is if a particular version of that package is required,
N:    in which case the dependency should include the version.
N:
N:    Refer to Debian Policy Manual section 4.2 (Package relationships) for
N:    details.
N:
N:    Severity: important, Certainty: certain
N:
N:    Check: fields, Type: binary, udeb, source
N: