lldpd: fix error "sh: XXXms: bad number"

lldpd: fix error "sh: XXXms: bad number"

from commit 3ce909914a12647bec52bcee0a162dd6d158a4f6

The lldpd man page says that "configure lldp tx-interval" can
specify an interval value in milliseconds by appending a "ms" suffix to
the figure. Thus mandating string handling, and not integer comparison.

Tested on 22.03.5

Signed-off-by: Paul Donald <newtwen@gmail.com>
(cherry picked from commit 79ee4cb0397eb9eae34369e33d66b5a1d837a652)
Link: https://github.com/openwrt/openwrt/pull/15299
Signed-off-by: Robert Marko <robimarko@gmail.com>