p54: eliminate warning for uninitialized variable 'tim_len'

drivers/net/wireless/p54/p54common.c: In function ‘p54_tx’:
drivers/net/wireless/p54/p54common.c:1058: warning: ‘tim_len’ may be used uninitialized in this function

Signed-off-by: John W. Linville <linville@tuxdriver.com>