MIPS: BCM47XX: trim the nvram values for parsing

Some nvram values on some devices have a newline character at the end
of the value, that caused read errors. Trim the string before reading
the number.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4745/
Signed-off-by: John Crispin <blogic@openwrt.org>