i3c: master: Remove set but not used variable 'old_i3c_scl_lim'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/i3c/master/i3c-master-cdns.c: In function 'cdns_i3c_master_do_daa':
drivers/i3c/master/i3c-master-cdns.c:1137:16: warning:
 variable 'old_i3c_scl_lim' set but not used [-Wunused-but-set-variable]

It never used since introdution in commit
  acfab7d324b2 ("i3c: master: Add driver for Cadence IP")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>