staging: et131x: Fix endian bugs in et131x_get_regs()

et131x_get_regs() calls et131x_mii_read(), passing the address of a u32
which is cast to a (u16 *). This works fine for little endian systems,
but not for big endian. Change so that the types are cast, not pointers
to the types.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>