libertas: remove two libertas sparse warning

Johannes Berg detected this two sparse warnings:

drivers/net/wireless/libertas/cmd.c:609:16: warning: cast to restricted __le16
drivers/net/wireless/libertas/cmd.c:611:16: warning: cast to restricted __le16

... but cmd.minlevel is "s8", so we can access it directly and hope
for the sign-extension-code in the compiler to convert that to the
"s16" type.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>