drivers/net: fix mislocated headers in cxgb4/l2t.c

For some reason three #include <linux/...> are buried way
down in the file.  Since the inclusion of module.h is one
of them, the inclusion comes after use of EXPORT_SYMBOL
which will cause warnings about implicit declarations.
Relocate all the headers to the top.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>