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. The max expected
value is 0xF, so now it should be possible to read decimal form ("15")
and hex form ("0xF").

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>