drivers/rtc/rv3029.c: fix GCC 4.6 build warnings

Fix:
rv3029.c: In function 'rtc_set':
rv3029.c:98:6: warning: variable 'ret' set but not used
[-Wunused-but-set-variable]
rv3029.c: In function 'set_eere_bit':
rv3029.c:131:6: warning: variable 'ret' set but not used
[-Wunused-but-set-variable]
rv3029.c: In function 'wait_eebusy':
rv3029.c:149:9: warning: variable 'ret' set but not used
[-Wunused-but-set-variable]
rv3029.c: In function 'rtc_reset':
rv3029.c:165:6: warning: variable 'ret' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>