brcm47xx: bgmac: allow bigger et_swtype nvram variable

Without this patch it is impossible to read et_swtype, because the 1
byte space is needed for the terminating null byte. Now it should be
possible to read decimal and hex vars of max 8 bit.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37999 3c298f89-4303-0410-b956-a3cf2f4a3e73