brcmsmac: Add helper function for updating txavail count
brcmsmac: Add helper function for updating txavail count

Use this helper function rather than open-coding the same calculation in
multiple places.

Signed-off-by: Seth Forshee <ead5052bef9b6b56b51f87cdc5b831be775ee6fb@canonical.com>
Reviewed-by: Pieter-Paul Giesberts <d101f7ec36e185d305862a92a05a9a0c1b62aa38@broadcom.com>
Reviewed-by: Arend van Spriel <06447fbe43693466d3293d0b5eaeeb2efd3d7d3a@broadcom.com>
Tested-by: Daniel Wagner <77048152ae2a79b9373e83a1adb46672db431ef3@monom.org>
Signed-off-by: John W. Linville <2a53bac7a5d324865ef46ec4c38b2c0fba1456b4@tuxdriver.com>

KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs