serial: sc16is7xx: convert bitmask definitions to use BIT() macro

Now that bit definition comments were cleaned up, convert bitmask
definitions to use BIT() macro for clarity.
Convert SC16IS7XX_IIR_ID_MASK to use GENMASK() macro -
- while at that, realign comments.
Compose SC16IS7XX_LSR_BRK_ERROR_MASK using aforementioned constants,
instead of open-coding it, and remove now unneeded comments.
Signed-off-by: Lech Perczak <lech.perczak@camlingroup.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/8b45a01e-7cc5-4d53-b467-c6680bc51ef4@camlingroup.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>