POP ROCK Motels: ''Little Robbers.'' Capitol ST-12288 - Martha Davis, guitarist-singer-songwriter for The Motels, confirms with this album that she is one of today's top women in the rock scene. The fast-paced, slightly new-wave-influenced sound of this group hits its mark. The title track makes you want to strut down Madison Avenue. Compared with their previous efforts, this album is remarkably diverse in its themes. ''Monday Shutdown'' is a little rocker about the frightening, for some, thought of going to work on Monday. ''Isle of You'' is a funky piece with a touch of reggae, complete with short, lively guitar chords and raspy vocals. The haunting love song ''Suddenly Last Summer'' highlights Miss Davis's singing voice, as well as her skill at writing smooth, flowing melodies. Besides this saddening love song, the album has but two other slow tunes, ''Footsteps'' and ''Tables Turned.'' Both purvey a sense of loneliness and sense of loss that is in contrast to the generally upbeat feel of the LP.