rtlwifi: rtl8192c-common: Fix two typos

In this driver, two variables are masked by one quantity, and then tested
against a second number with more bits that the mask. Accordingly, the
test always fails. To minimize the possibility of such typos, a symbolic
definition of the mask is created and used.

The separate load and mask operations are also combined into a single
statement.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: David Binderman <dcb314@hotmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>