package/linux-firmware: fix typo in firmware file for cxgb4

Trivial inversion b/g: cxbg4 -> cxgb4

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Jeremy Kerr <jk@ozlabs.org.>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>