finding unpredictable and sporadic absences during probationary period of employment indicated that employee could not fulfill an essential function of his employment - presence on the job - that no reasonable accommodation could be made to cure the unpredictable nature of the absences, "[r]equiring the [employer] to accommodate such absences would place upon the [employer] the burden of making last-minute provisions for [plaintiff's] work to be done by someone else" and thus, summary judgment in favor of employer was appropriate

affirming the granting of summary judgment to the defendant employer and holding that an employee with a history of sporadic, unpredictable absences is not otherwise qualified under the ADA