rt2x00: Always inline rt2x00pci_enable_interrupt

This allows the compiler to perform the necessary bitfield calculations
during compile time instead of run time and thus reduces the number of
instructions to run during each tasklet invocation. This should improve
performance in the RX hotpath.

This comes at the cost of a slight increase in the module size (for
example rt2800pci):

Before:
   text    data     bss     dec     hex filename
  14133     832       4   14969    3a79 drivers/net/wireless/rt2x00/rt2800pci.ko

After:
   text    data     bss     dec     hex filename
  14149     832       4   14985    3a89 drivers/net/wireless/rt2x00/rt2800pci.ko

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>