GCC4.6: Convert various empty macros to inline functions

Fix the following gcc4.6 problems:

cmd_date.c: In function ‘do_date’:
cmd_date.c:50:6: warning: variable ‘old_bus’ set but not used
[-Wunused-but-set-variable]
asix.c: In function ‘asix_init’:
asix.c:317:6: warning: variable ‘rx_ctl’ set but not used
[-Wunused-but-set-variable]
usb.c: In function ‘usb_parse_config’:
usb.c:331:17: warning: variable ‘ch’ set but not used
[-Wunused-but-set-variable]
usb.c: In function ‘usb_hub_port_connect_change’:
usb.c:1123:29: warning: variable ‘portchange’ set but not used
[-Wunused-but-set-variable]
usb.c: In function ‘usb_hub_configure’:
usb.c:1183:25: warning: variable ‘hubsts’ set but not used
[-Wunused-but-set-variable]
usb_storage.c: In function ‘usb_stor_CB_reset’:
usb_storage.c:466:6: warning: variable ‘result’ set but not used
[-Wunused-but-set-variable]

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>