mt7601u: remove redefinition of GENMASK

Remove redefinition of GENMASK which should not be there
in the upstream version of the code.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dinan Gunawardena <dinan.gunawardena@netronome.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>