ath9k_htc: Implement multiple register write support

This patch adds support for writing multiple registers
in a single USB command.

Specific calls from the HW code that performs multiple
register writes would be modified to make use of this
in subsequent patches.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>